Printing using an electrophotographic system has been markedly developed in recent years not only in a printer for PC (personal computer), facsimile or copying machine, but also in the so-called on-demand printing field which realizes printing with many kinds of products and a small lot size, variable information printing, etc. In recent years, accompanying with improvement in printing speed and image quality, it is now started to utilize the system in the region in which a large number of printing is carried out as in the conventional offset printing or gravure printing.
The electrophotographic printing is a printing system not using a printing plate so that it has a merit that variable information can be treated. On the other hand, offset printing or gravure printing cannot treat variable information, but are suitable for high quality printing with inexpensive and a large amount. Thus, in the electrophotographic system, for opposing to the offset printing or gravure printing, technical development has been carried out in the points of a printing machine, toner and recording sheet to obtain high image quality, high speed printing, saving electric power, and low cost. Also, there are demands to further improve toner fixability, transferability, color reproducibility, etc.
Among electrophotographic printing systems, a dry electrophotographic system is a system represented by a copying machine for official use, etc., and as a toner for forming an image, a solid powder toner comprising a pigment and a synthetic resin is used. Printing is carried out by an image forming method which comprises adsorbing a toner to an electrostatic image formed on a light-sensitive material, transferring the toner to a material to be transferred, and heating the material to be transferred to fix the toner thereon. However, when the toner is made fine for the purpose of high quality image in this system, the toner is likely dissipated to surroundings, whereby there are problems in health when the toner is inhaled by a person and further a printed material is contaminated. Due to the limitation of making the toner fine, there are pointed out the problems that fine dot printing cannot be carried out, a consumed amount of the toner is large, an image is swelled and looks unnatural as a printed material, etc. In addition, there are problems that a recording sheet becomes waving due to high temperature fixing, and a consumed electric power becomes large. As a recording sheet to be used in the dry electrophotographic system, other than usual paper, there are recording sheets in which a porous toner receiving layer is provided on a substrate as disclosed in JP S63-33749A, JP H7-81214A, JP 2006-227473A, JP 2007-127767A, JP 2007-240826A, JP 2008-70422A, etc.
On the other hand, the wet electrophotographic system is an extremely promising system. This is because the toner is dispersed in a liquid medium in the wet electrophotographic system, so that no problem in dissipation of powder occurs, a size of the toner can be made fine about 1/10 as compared with that of the dry electrophotographic system, i.e., fine dot printing can be carried out, no problem occurs in weather resistance since a pigment can be used as a coloring agent, swelling of an image is not so significant whereby the quality of the printed material is close to that of the offset printing, etc.
In an image forming method of the wet electrophotographic system, a wet toner is adsorbed onto an electrostatic image formed on a light-sensitive material, a solvent contained in the wet toner is removed prior to transfer the image on a recording sheet, and then the toner is transferred onto the recording sheet to form an image. Or else, after transferring a toner containing a solvent onto a recording sheet, the solvent contained in the toner is removed to form an image. When the solvent in the wet toner is to be removed prior to transfer onto the recording sheet, a blanket roll heated to about 60 to 120° C. is utilized. In this case, a wet toner image containing a solvent on a light-sensitive material is firstly transferred onto a blanket roll, and the solvent is removed by the heat of the blanket roll. Next, the toner is heat-fused on the blanket roll to have stickiness and transferred onto a recording sheet to form an image. On the other hand, when the solvent in the wet toner is to be removed after transferring onto a recording sheet, the recording sheet on which the wet toner containing the solvent has transferred is heated to remove the solvent on the recording sheet, and the toner is heat-fused thereon to fix it to the same.
In the method in which an image is transferred from a heated blanket roll to a recording sheet, it is not necessary to directly heat the recording sheet, so that heat resistance of the recording sheet is irrelevant to form an image. Thus, a film or a substrate for photographic printing with low heat resistance can be used as a substrate for the recording sheet, so that it is preferred to broaden the width of choices of the recording sheet.
In the method in which an image is transferred from a heated blanket roll to a recording sheet, no adsorption action due to static electricity is utilized when an image is transferred onto the recording sheet. Transfer of the image of this case is carried out so that an adhesiveness of the wet toner and the recording sheet is higher than that of the wet toner and the blanket roll. That is, for transferring the toner onto the recording sheet effectively, a transferability of the wet toner from the blanket roll to the recording sheet is important. When an adhesive ability of the toner itself is heightened than required for the purpose of heightening adhesiveness between the recording sheet and the toner, fixing to the sheet becomes good. However, adhesiveness between the blanket roll and the toner is also heightened simultaneously whereby the toner becomes difficultly peeling from the blanket roll so that the adhesive ability of the toner cannot be heightened than required. Thus, in the case of a wet electrophotographic system printing in which an image is transferred from the heated blanket roll to the recording sheet, there are defects that sufficient toner fixing strength cannot be obtained when printing is carried out, or toner particles are not sufficiently transferred onto the sheet. If sufficient toner fixing strength cannot be obtained, the image on the printed material is markedly deteriorated by handling thereafter. Also, if transfer of the toner is not sufficient and incomplete, a part of the toner remains on the blanket roll surface, and it appears in an image printing thereafter as a remaining image (ghost) to markedly lower the image quality. Occurrence of such a ghost also occurs, for example, when continuous printing is carried out, by the reason that the blanket roll surface is suffered from bad effects at a non-image portion at which the blanket roll surface and the recording sheet are directly contacted, and wet toner transferability of an image to be printed later is lowered and ghost occurs in some cases.
Thus, as a means to obtain a sufficient toner fixing strength, there is a method in which a fixing agent is coated onto a recording sheet. For example, a wet electrophotographic printing machine manufactured by Hewlett Packard has a treatment function called “Sapphire treatment”, which is to cover the recording sheet with polyethyleneimine, but there are problems that yellowing occurs during storage of the recording sheet, or toner is peeled off when the printed material is handled after printing. Also, to provide wet electrophotographic printing suitability without employing such a treatment, it has been known a recording sheet as disclosed in, for example, JP H10-20537A (Patent Literature 1), JP 2003-173038A (Patent Literature 2), and JP 2004-503805A (Patent Literature 3), etc., in which a coating layer containing a polyamide series polymer having an amino group as the terminal group or a poly-ethyleneimine series polymer had been provided on a substrate. However, it is extremely difficult to obtain sufficient toner fixability or transferability.
As a means to improve fixability of the wet toner or transferability of the toner on a blanket roll surface without using a fixing agent, there have been proposed a method in which calcium carbonate or kaolin which easily absorbs a solvent contained in the wet toner is added to a toner receiving layer of a recording sheet as disclosed in, for example, JP 2004-77667A (Patent Literature 4), JP 2005-250168A (Patent Literature 5), WO 2004/49074A (Patent Literature 6), JP 2009-169408A (Patent Literature 7), etc. These methods act as a method for improving toner fixability but are not sufficient, so that further improvement has been desired. Also, an example in which a toner receiving layer is constituted by a polyvinyl alcohol is disclosed in JP 2005-17844A (Patent Literature 8), an example in which it is constituted by a polyvinyl alcohol and colloidal silica is disclosed in JP H9-114122A (Patent Literature 9), and an example in which it is constituted by various kinds of latex resins is disclosed in JP 2006-39435A (Patent Literature 10), JP 2004-258339A (Patent Literature 11), etc. However, even when these toner receiving layers are employed, there is a sense of incongruity (gloss unevenness) in glossy feeling between an image portion and a non-image portion, whereby it is not reached to a sufficient level as photographic image quality.
Also, various kinds of copolymer latexes such as styrene-butadiene series, acrylic series, polyvinyl acetate, ethylene-vinyl acetate, etc., and various kinds of starches, which are used as a binder in the toner receiving layer containing a pigment of the recording sheet for the wet electrophotographic system, do not necessarily have a high binding force with the pigment, so that there are problems that peeling, causing crack or crack by folding of the toner receiving layer occurs depending on the kind of the pigment or an added amount thereof. In particular, when inorganic fine particles having a small particle size are used as a pigment, the above-mentioned problem becomes more remarkable.